This was broken since e8baaba044689b75ef4f36ebc5a93b7462c23cb2,
because on the tracker, a DNS lookup for "tracker" returns ::1 due to
nss_myhostname. This apparently confused it.
You can now run a test in the nixos/tests directory directly using
nix-build, e.g.
$ nix-build '<nixos/tests/login.nix>' -A test
This gets rid of having to add the test to nixos/tests/default.nix.
(Of course, you still need to add it to nixos/release.nix if you want
Hydra to run the test.)
